1. Field of the Invention
The invention is related to an offshore structure used for the exploration and production of oil or gas or for some other purpose.
2. General Background
In the offshore drilling industry it is common to use a fixed structure such as a jacket or tower. The jacket or tower is normally held in place by pilings driven into the sea floor through the legs of the structure and possibly also through a template on which the tower is placed. Exploration and development of hydrocarbons and minerals in deeper waters requires taller towers. The larger towers are more expensive to fabricate and require equipment with increased lifting and weight bearing capacity to accommodate the extra weight of the larger towers. The larger towers also have a tendency to present stability problems during installation.
U.S. Pat. No. 4,793,739 teaches a compliant pile system for supporting a concrete structure so that the structure becomes compliant. The fixation of the sleeves is at the lower end at the seabed and the piles can be at the upper portion and the primary function of the pile system is to provide stability and to support it vertically. It is stated that the pile system will increase the sway period. However, the period will be increased only if the stiffness of the concrete structure is reduced. Adding piles will in general increase the overall stiffness of the platform and will decrease the period of oscillation. The concrete structure can be provided with a single rotation shell at the bottom. The piles act to restrain the concrete structure by bending action over the portion above the seabed. The piles provide vertical support.
U.S. Pat. No. 4,721,416 teaches a pile system supported by guides attached to a jacket structure. The fixation of the piles is at the sea bed and at the guides. They are elongated to support the topsides. The piles act as the legs of the structure and the jacket structure only provides the bracing necessary to transfer shear forces between the piles so that the offshore jacket can act in bending. The jacket structure does not support any vertical loading. The piles do not extend above the framing, so that they are not braced over a significant length.
U.S. Pat. No. 4,969,604 teaches a compliant pile assembly for use on a compliant tower system and concerns the use of drive piles and flex piles. The drive piles are driven into the seabed and extend sufficiently to permit a sleeve attached to the side of the flex piles to engage with the drive piles and thereby be attached to them. The flex pile is aligned eccentrically with the corresponding drive pile to which it is attached. To minimize the bending effects, the piles have to be arranged symmetrically about the leg of the tower. The tower supports the topsides but is not provided with a foundation so that the vertical loads are transferred to the flex piles and through the eccentric connection to the drive piles. This also applies to horizontal loads such as environmental forces. The flex piles provide support over a limited portion of the tower and do not directly resist loads from a deck or topsides.
The present art does not teach a structure that permits an efficient distribution of loads between the piles and tower. This is due to the unsupported tower and the transfer of load from the tower to the piles through an eccentric connection.